Encapsulation of various materials has hitherto been performed in many industries and technical fields. In the printing, paint and ink industries, encapsulation of pigments or dyes has been in practical use in large numbers. Further, also in the pharmaceutical and agricultural chemical fields, encapsulation of drugs has been in heavy usage, in order to retard their effect or to decrease their toxicity. As encapsulation methods, there have generally been known a phase separation method (a coacervation method), a submerged drying method (an interfacial precipitation method), a spray drying method, a pan coating method, a submerged curing coating method, an interfacial polymerization method, an interfacial inorganic reaction method and an in-situ polymerization method. However, these methods have had the problems that a core material is limited, that it is difficult to freely design the thickness of a shell layer (a coating layer for the core material), that it is difficult to encapsulate one core material, that it is difficult to freely design a functional group on a capsule surface, that it is not easy to produce particles having a homogeneous surface state, and that encapsulation in a nano-order is not easy.
Further, in an ink jet recording process comprising ejecting ink droplets from a fine nozzle head to form letters or figures on a surface of a recording medium such as paper, an aqueous pigment ink in which a pigment is dispersed in water has recently been used because of its excellent water resistance and light resistance. As such an aqueous pigment ink, there has been generally used in many cases one in which the pigment is dispersed in an aqueous dispersing medium with a dispersant such as a surfactant or a polymer dispersant. When the pigment is dispersed with the dispersant as described above, the dispersant is merely adsorbed on surfaces of the pigment particles. Accordingly, in the ink jet recording process in which strong shear force is applied to the pigment particles, the dispersant adsorbed on the surfaces of the pigment particles is eliminated in some cases, whereby dispersibility of the pigment ink deteriorates to cause deterioration in ejection stability (the characteristic of being ejected from a recording head in a definite direction) in some cases. Further, the pigment ink in which the pigment is dispersed in the aqueous dispersing medium with the dispersant such as the surfactant or the polymer dispersant is liable to cause the adsorption and desorption of the dispersant. When the ink is stored for a long period of time, the dispersion is liable to become unstable.
On the other hand, in order to improve fixability of a pigment contained in a pigment-based ink jet ink to a recording medium, a technique using an encapsulated pigment in which colorant particles are coated with a polymer has been known.
There have been proposed encapsulated fine pigment particles (for example, see patent documents 1, 2 and 3), and pigment particles on surfaces of which a polymer is graft polymerized (for example, see patent documents 4 to 7). Further, there has been proposed a method for encapsulating a hydrophobic powder with an amphiphilic graft polymer (for example, see patent document 8). However, the use of a previously polymerized polymer has raised the problem that the particle size after encapsulation becomes too large.
In addition to the above-mentioned proposals, there have been proposed inks using a pigment coated with a resin having film forming properties at room temperature by phase inversion emulsification (for example, see patent documents 9 to 17), and inks using a pigment coated with an anionic group-containing organic polymer compound by acid precipitation (for example, see patent documents 18 to 27).
Further, there have been proposed inks using a polymer emulsion in which fine polymer particles and a colorant are impregnated by phase inversion emulsification (for example, see patent documents 28 to 33). However, in the colorant obtained by phase inversion emulsification or acid precipitation, the polymer adsorbed on the pigment particles is also sometimes eliminated and dissolved in the ink, depending on the kind of organic solvent such as a penetrant used in the ink, so that dispersion stability and ejection stability of the ink, image quality and the like have been insufficient in some cases.
Patent Document 1: JP-B-7-94634
Patent Document 2: JP-A-8-59715
Patent Document 3: JP-A-2003-306661
Patent Document 4: JP-A-5-339516
Patent Document 5: JP-A-8-302227
Patent Document 6: JP-A-8-302228
Patent Document 7: JP-A-8-81647
Patent Document 8: JP-A-5-320276
Patent Document 9: JP-A-8-218015
Patent Document 10: JP-A-8-295837
Patent Document 11: JP-A-9-3376
Patent Document 12: JP-A-8-183920
Patent Document 13: JP-A-10-46075
Patent Document 14: JP-A-10-292143
Patent Document 15: JP-A-11-80633
Patent Document 16: JP-A-11-349870
Patent Document 17: JP-A-2000-7961
Patent Document 18: JP-A-9-31360
Patent Document 19: JP-A-9-217019
Patent Document 20: JP-A-9-316353
Patent Document 21: JP-A-9-104834
Patent Document 22: JP-A-9-151342
Patent Document 23: JP-A-10-140065
Patent Document 24: JP-A-11-152424
Patent Document 25: JP-A-11-166145
Patent Document 26: JP-A-11-199783
Patent Document 27: JP-A-11-209672
Patent Document 28: JP-A-9-286939
Patent Document 29: JP-A-2000-44852
Patent Document 30: JP-A-2000-53897
Patent Document 31: JP-A-2000-53898
Patent Document 32: JP-A-2000-53899
Patent Document 33: JP-A-2000-53900